Ergosterol is a sterol compound primarily found in the cell membranes of fungi, yeast, and some protozoa, where it maintains membrane fluidity and integrity similar to cholesterol in animals. It serves as the biological precursor to vitamin D2 (ergocalciferol) through UV-induced photochemical conversion. Structurally, it features a tetracyclic steroid nucleus with three double bonds (positions 5-6, 7-8, and 22-23) and a methyl group at C24. Ergosterol is abundant in mushrooms and is commercially extracted for vitamin D fortification in foods and supplements. Certain antifungal agents, such as amphotericin B, target ergosterol by binding to it and disrupting fungal membranes. It is lipophilic and plays roles in plant-fungal interactions and biotechnology applications.